1. Field of the Invention
This invention generally relates to methods and compositions for reducing sulfur levels in flue gases generated by hydrocarbon catalytic cracking units, coal and/or oil-fired power plants and chemical manufacturing facilities.
2. Description of the Prior Art
Sulfur is often a component of the feedstocks processed by many industrial facilities. It also is found in the fossil fuels used to power and/or create process heat for such facilities. Hence, the sulfur contained in such materials has the potential to become an atmospheric pollutant--especially when it takes the form of those sulfur oxide gases that become a part of the flue gases emitted from such facilities. Such emissions are particularly harmful to the atmosphere and, hence, are the subject of extensive governmental regulation. One of the most commonly used methods for preventing release of these sulfur oxide gases into the atmosphere is to capture them through use of compounds that have an ability to absorb them.
For example, in the case of recovering sulfur oxide gases from flue gases generated by fluid catalytic cracking units (FCC units) used to crack petroleum feedstocks, microspheroidal catalyst particles having chemical activities toward sulfur oxide gases are circulated in admixture with the microspherical particles used to carry out the petroleum cracking function. These hydrocarbon cracking catalyst particles are often referred to as "bulk" or "FCC" catalysts while the sulfur catalyst particles are often referred to as "SO.sub.x additives." During the hydrocarbon cracking process, a coke-like material that also contains a sulfur component--if sulfur is contained in the petroleum feedstock--is deposited on the SO.sub.x additive particles as well as on the FCC catalyst particles. Both kinds of particles, and hence the coke and sulfur deposited on them, are carried from the FCC unit's reactor to its catalyst regenerator. Here, the coke, and whatever sulfur that is contained in that coke, is "burned off" both kinds of catalyst particles. The sulfur component of such coke/sulfur deposits forms sulfur oxide gases (e.g., sulfur dioxide and sulfur trioxide which are often collectively referred to as "SO.sub.x " gases). Unless captured, these SO.sub.x gases would be emitted to the atmosphere along with other flue gases given off by the catalyst regenerator (e.g., carbon monoxide, carbon dioxide, nitrous oxides, etc.). In other kinds of industrial facilities (e.g., coal-fired power plants and certain chemical manufacturing plants), SO.sub.x additives are usually employed in the form of larger particles such as pellets that are not circulated throughout the facility in the form of microspheroidal particles, but rather are used in so-called "fluid bed" or "fixed bed" catalyst systems. In such systems, these catalyst pellets perform their SO.sub.x additive functions in a more localized region--as opposed to being circulated throughout the entire unit. These fixed bed and fluid bed systems are usually provided with so-called "swing reactors" which provide more than one fluid bed or fixed bed so that at least one bed can be used to capture SO.sub.x while at least one other bed is being regenerated. Be these swing reactor configurations as they may, they too produce sulfur-containing flue gases. Thus, even though the sulfur contained in the fossil fuels used to power electrical power plants and/or provide process heat for chemical manufacturing facilities is converted into SO.sub.x gases in a manner somewhat different from that of FCC units, the end result is the same; unless captured, their SO.sub.x emissions can and do enter, and pollute, the atmosphere.
Many materials have been used to prevent, or at least reduce, SO.sub.x emissions from all such industrial facilities. The SO.sub.x absorbing component of these additives is normally a metal oxide of one kind or another. Generally speaking, these metal oxides carry out their SO.sub.x capturing function by forming metal sulfates when they are exposed to SO.sub.x -containing gases, especially under high temperature conditions. A more complete identity of these metal oxides will be provided in later portions of this patent disclosure.
Regardless of their identity, however, regeneration of "sulfated" SO.sub.x additive particles usually involves converting them from their "contaminated" metal sulfate forms back to their "uncontaminated" metal oxide forms. For example, in the case of a FCC unit, the metal sulfate forms of the SO.sub.x additive (that are produced in the catalyst regenerator unit) are circulated, in admixture with regenerated hydrocarbon cracking catalyst, from the catalyst regenerator unit back to the FCC unit's hydrocarbon cracking reactor zone. Here, the petroleum feedstock is cracked and the sulfur components of the SO.sub.x additive particles are converted to hydrogen sulfide gas by the hydrocarbon/ hydrogen rich atmosphere existing in such reactor zones. As a consequence of this, the metal sulfate component of a SO.sub.x additive is reduced to its metal oxide form and, thus, is made ready for subsequent reuse in the catalyst regenerator. The hydrogen sulfide gas produced in the FCC reactor unit is eventually captured and ultimately reduced to elemental sulfur in ways well known to the chemical engineering arts.
In the case of fluid bed or fixed bed catalyst systems such as those used to control SO.sub.x emissions from power plants, the SO.sub.x additive is usually regenerated by passing a hydrocarbon-containing gas through a SO.sub.x additive bed during a swing reactor regeneration cycle. This operation also serves to convert those metal sulfates contained in the used, SO.sub.x additive pellets back to their metal oxide forms. Methane, propane, and butane gases, as well as hydrogen gas itself, are used to carry out the regeneration of such SO.sub.x additives in these fixed bed or fluid bed systems.
Regardless of the exact nature of the industrial process being carried out, and regardless of the physical size of the SO.sub.x additive materials being used, and regardless of the method used to regenerate such materials, any given SO.sub.x additive system must perform at least three basic functions with respect to the sulfur oxide gases they seek to capture. First, these SO.sub.x additive systems must oxidize SO.sub.2 to SO.sub.3 ; second, they must absorb the SO.sub.3 once it is formed; and third, they must be able to "give up" the captured SO.sub.3 in order to be regenerated. The need to convert SO.sub.2 to SO.sub.3 follows from the fact that very few materials are capable of both absorbing SO.sub.2 gas and withstanding the high temperature conditions where the SO.sub.2 is created. There are, however, many materials (e.g., various metal oxides) that are both capable of absorbing SO.sub.3 and withstanding the high temperature environments where it is formed.
In most cases, these metal oxides are bivalent and/or trivalent metal oxides. For example, magnesia and/or alumina have been widely employed as SO.sub.x additives in many different kinds of hydrocarbon catalytic cracking systems. By way of example, U.S. Pat. Nos. 4,423,019; 3,835,031; 4,381,991; 4,497,902; 4,405,443 and 4,369,130 teach SO.sub.x catalytic and/or absorbent activities for various metal oxides.
The prior art also has long recognized that certain metals (e.g., cerium, vanadium, etc.) and their oxides (e.g., ceria, vanadia) can be employed in SO.sub.x additive systems in order to improve their ability to oxidize SO.sub.2 to SO.sub.3. Indeed, it might even be said that, to a very large degree, the prior art with respect to using metal oxide materials as SO.sub.x oxidants and/or absorbents has, for the past several decades, largely concerned itself with finding better ways of associating various catalytically active metals (e.g., cerium, vanadium, etc.) with all manner of metal oxide materials in order to enhance the resulting material's SO.sub.x catalyzing and/or absorbing capabilities.
Some metal oxides also are known to improve the "release" of the sulfur component of "used" SO.sub.x additives when it comes time for them to be reduced back to their metal oxide forms. For example, U.S. Pat. No. 4,589,978 ("the '978 patent") teaches SO.sub.x transfer catalysts based upon the use of rare earth metals such as cerium and lanthanum. The '978 patent also teaches use of alumina to absorb SO.sub.3 by forming aluminum sulfate in circumstances wherein the alumina is employed in the form of a separate and distinct particle species that is used in admixture with other particles that contain a SO.sub.2.fwdarw.SO.sub.3 oxidant. To these ends, the '978 patent states: "The SO.sub.x transfer catalyst of the present invention preferably includes a metal oxide such as alumina to absorb SO.sub.3 as sulfate. The alumina may be circulated as a separate particle or used as a support for the rare earth component. Preferably, the alumina is an active form with high surface area, which includes synthetic alumina in gamma, theta, etc. forms as well as natural aluminas." We would specifically note here that the '978 patent at least, in principle, recognizes that its alumina SO.sub.3 -absorbing component "may be circulated as a separate particle . . . ". We also would note that the above-quoted phrase goes on to say that its alumina may be "used as a support for the rare earth component." Later, the 1978 patent goes on to say that its ". . . alumina and rare earth components can be further supported on an inert support or matrix which does not react with SO.sub.2 or SO.sub.3 to form sulfate. The supports for the alumina and rare earth oxidation component may be selected from silica, silica/alumina, zeolites, kieselguhr, celite or alumina." We have quoted these passages from the '978 patent because, in some ways, the teachings of this patent reference define certain "points of departure" that help to establish and define the borders of the novel aspects of the invention described in this patent disclosure. Therefore, the teachings of the '978 patent are incorporated herein in their entirety.